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In recent years functional MRI studies of human reward learning have signicantly advanced our understanding of how the brain represents rewards and learns reward associations. The studies presented in this thesis build on this work to further char- acterize the functional contributions of regions such as the orbitofrontal cortex and ventral striatum, with a focus on understanding how neural activity relates to behav- ior, specically in terms of valuation and decision-making.

Attractive faces have been shown to be a form of visual reward, suggesting that they should aect behavior and neural activity in a manner similar to other types of reinforcers. In Chapter 1 we tested this hypothesis and demonstrated that attractive faces can act as reinforcers in a classical conditioning paradigm. The aective pleas- antness of a set of neutral visual cues increased as a result of repeated pairings with attractive, compared to unattractive, female faces. We found that reward prediction errors in the ventral striatum were engaged during learning, as has been found for other types of reinforcers such as food, pain, and money.

The change in valuation for cues paired with attractive female faces was especially pronounced in male subjects, while female subjects did not show a similar eect in response to male faces. Interestingly, in male subjects prediction error responses were strongest for female faces, and prediction error responses in female subjects were strongest for male faces. This suggests that learning takes place similarly in the brains of male and female subjects, but is expressed dierently at the behavioral level. An avenue for future study would be to employ dierent behavioral probes to

investigate if and how female subjects express this learning.

More generally the results of this study are relevant for marketing studies, which have shown that the presence of an attractive female model in an advertisement can inuence customer perception of a product [101, 102]. Our ndings suggest that classical conditioning mechanisms may contribute to this eect.

Pavlovian cues elicit passive responses but can also exert control over instrumen- tal responding. In Chapter 2 we presented the rst investigation into the neural mechanisms by which Pavlovian cues exert control over human decision-making. We showed that a Pavlovian cue predictive of a specic liquid reward can bias action choice towards responses associated with the same liquid reward. We found that a region of ventrolateral putamen was relatively suppressed when subjects made choices incompatible with the Pavlovian cue. While lesion studies in animals have shown that regions of ventral striatum are necessary for the expression of Pavlovian-instrumental transfer eects [83], this study is the rst to show the dynamics of neural activity involved in outcome-specic transfer.

Current theories propose that transfer mechanisms are mediated by stimulus- outcome and outcome-response associations [79]. Our results t nicely with this theory: we interpret our nding of a relative suppression when an incompatible cue is chosen as related to the suppression of an outcome-response association stimulated by the Pavlovian cue.

We note that the regions we found to be involved in outcome-specic transfer are distinct from those found in a recent fMRI study on general transfer eects, in which a Pavlovian cue enhances response vigor rather than inuencing decision-making per se [86]; this mirrors the dissociation in neural circuitry found in animal studies of general and specic transfer eects [83, 84, 85]. However, it will be important in future studies to demonstrate both general and specic transfer eects in the same paradigm.

Several interesting features of Pavlovian-instrumental paradigms have been iden- tied in the animal literature, of particular interest is the eect of reinforcer devalua- tion. It has been shown that in certain situations devaluing the reinforcer associated

with the Pavlovian cue does not suppress the expression of transfer eects in animals [148, 79]. This nding has clear parallels with addictive behaviors, in which environ- mental cues trigger drug-seeking, even when the outcome has known aversive eects.

Few studies have directly probed the links between transfer eects and addictive be- haviors [150], but this line of research could prove important in understanding the neurophysiological underpinnings of addiction. One potential avenue for treatment could involve training subjects to suppress regional activity in order to successfully avoid making choices associated with environmental cues [100, 160].

A further extension of the work presented in chapters 1 and 2, related to the impact of attractive faces used in advertising, would be to test whether cues associated with attractive faces can exert control over instrumental behaviors, as has been shown with other types of reinforcers [150, 86].

In Chapters 3 and 4 we investigate how provision of reward can inuence neural plasticity: we trained human subjects to activate specic brain regions in order to earn reward. We demonstrated that a shaping procedure in which subjects were given monetary rewards for making improvements on their past performance was successful in training an increase in dierential activity across sessions. This technique presents an alternative to standard bio/neurofeedback approaches and may prove useful in many clinical and research applications.

In the study described in Chapter 3 we successfully trained subjects to dieren- tially activate regions of motor cortex related to hand and foot movements, in absence of overt movements. We investigated behavioral eects of this learning, and showed that reaction times in a cued response task were dierentially aected by presentation of the learned cues.

A primary motivation for developing this technique was to condition neural activ- ity in emotional brain regions, in order to study the causal eects of elevated activity on behavior. In the study described in Chapter 4 we trained subjects to activate me- dial orbitofrontal cortex (mOFC) activity and probed the impact of this training on an aective judgment task. We demonstrated that subjects can improve at elevating mOFC activity on cue, and that elevated activity was associated with a positive bias

in aective evaluations. This study represents a signicant advance in our under- standing of how mOFC activity aects our perception of value, as previous imaging studies have been unable to establish this causal link.

Taken together, these studies advance our understanding of the functional con- tributions of ventral striatum and orbitofrontal cortex in inuencing decision-making and valuation, and suggest that applying associative learning techniques to real-time fMRI training can be a powerful method for characterizing the causal inuence of regional neural activity.

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